1. Field
The present invention relates to an ink.
2. Description of the Related Art
An inkjet recording method is a recording method of forming an image by applying ink droplets to a recording medium such as plain paper or a lustrous medium, and is rapidly spreading due to reduction of the cost on the method and improvement of recording speed. Also, with rapid spread of digital cameras in addition to image quality enhancement of an image obtained by the inkjet recording method, the inkjet recording method has become widely popular as a method for outputting an image comparable to a silver salt photograph.
As one problem with the inkjet recording method, there is illustrated a problem with image preservability of an obtained recorded product. In general, a recorded product obtained by the inkjet recording method involves the problem that its image preservability is poor in comparison with a silver salt photograph. More specifically, there is a problem that, when exposed to light, moisture, heat, or environmental gases existing in the air for a long time, the recorded product suffers deterioration of coloring materials on the recorded product, which is liable to cause color change or color fading of the image. An image formed by a black ink often undergoes change in color tone due to deterioration of coloring materials and is recognized to undergo particularly serious deterioration in light fastness. Hence, this has so far been recognized as a problem to be solved, and many proposals have been made.
For such problem, it has been proposed in JP-A-2009-30023 and JP-A-2009-185133 to improve light fastness of an image by using an ink having a structure showing excellent light fastness.
On the other hand, ink compositions to be used for forming the above-described color image are required so that a recorded product obtained by a combination of plural color ink compositions will not undergo color change or color fading during storage and that bronzing will not occur in addition to that each ink composition itself for each color has good color-forming properties.
With an ink set, when a particular ink composition has a seriously poor light fastness or ozone fastness than other ink compositions have, the color formed by the particular ink undergoes color fading or color change faster than other colors, leading to poor balance of color tone of the entire image. Therefore, it is preferred to improve light fastness and ozone fastness of respective ink compositions constituting an ink set and, at the same time, to synchronize the speed of deterioration such as fading of respective images formed by corresponding ink compositions as much as possible. In other words, it is preferred that respective inks constituting an ink set have excellent light fastness and excellent ozone fastness and that difference in light fastness and ozone fastness between the inks is small.
Also, in the case where a recorded image with a high optical density is formed, there has been a problem that crystals of dyes are precipitated on the surface of an image-receiving material as drying advances, leading to reflection of light by the recorded image to give metal luster, which is called bronzing phenomenon. This phenomenon is considered to be liable to occur as a result of enhancement of associative properties (aggregation properties) of dyes which is caused by decreasing water solubility of the dyes or introducing a hydrogen-binding amino group into the dye structure for the purpose of improving water resistance. Since light is reflected due to occurrence of bronzing phenomenon, not only optical density of the recorded image is reduced, but color hue of the recorded image becomes seriously different from desired color hue. Accordingly, suppression of bronzing phenomenon is one of important things required for an inkjet ink.